The so-called Hadfield steels or manganese steels owe their names to their British inventor, Sir Robert Hadfield in 1882, and are basically characterized by comprising an amount of manganese usually above 11% by weight, the ratio between carbon and manganese also being adjusted, such that the ratio by weight of manganese is usually in an order of eleven times the weight of carbon. These steels usually comprise 0.8-1.25% of carbon and 11-15% of manganese by weight in their basic composition.
Hadfield steels have a high impact strength and resistance to abrasion.
Hadfield steel reaches its properties of maximum hardness and ductility at about 12% by weight of manganese values.
These steels are non-magnetic and with low conductivity having the peculiarity that, among others, their impact performance improves with cold working. In this sense, the hardness of these steels increases up to three times the initial hardness after working under impact, which confers them a special usefulness for use thereof in determined applications, such as for example manufacturing of railway crossings, quarry parts or parts for cement manufacturing plants and in numerous applications in the scope of primary industry, such as in mining.
The manganese steels known as Hadfield steels have an elemental chemical composition which, according to the recommendations of the standard usually followed for the manufacture thereof, the United States ASTM A 128 standard, which are also found in the European prEN-15689-2007-ING standard, have the following basic chemical composition:                Carbon: 0.90 to 1.35%        Manganese: 11.00 to 14.00%        Silicon: 0.8% maximum        Phosphorus: 0.07% maximum        Sulphur: 0.05% maximum        
The starting hardness of this material is from 190 to 220 HB after a sudden and extreme quenching treatment at 1050° C., obtaining, according to prEN-15689-2007-ING standard, a greater tensile strength from 700 to 800 MPa, an elongation between 10 to 35%, a yield stress between 320 to 400 MPa and a resilience between 50 and 160 J.
As a general rule, the manganese content is not usually less than, and should not be much greater than, 11.00% given that in these cases, the wear resistance improves but ductility is seriously compromised. Furthermore, by exceeding this proportion, the price of the manufactured material is increased without significantly improving its mechanical characteristics. It is acknowledged that the suitable properties are obtained with a composition of 1.20% of C and 12.50% of Mn.
The existence of Hadfield steels which incorporate alloy elements such as V, Cr, Mo, Ti, Nb, N or Ce for the purpose of improving some of its properties is presently known. However, improving some of them is achieved to the detriment of another. Furthermore, these alloyed Hadfield steels usually have residual stresses greater than those of a conventional Hadfield steel since the additions cause changes in the crystallographic structure of the steel.
As is seen in Table 1, the addition of alloy elements generally entails an improvement of some of the mechanical properties.
TABLE 1Mechanical properties of different Hadfield steels tested according to theprEN-15689-2007-ING standard.TensileYieldstrengthstressElongationResilienceHardnessMaterial(MPa)(MPa)(%)(J)(HB)Basic700-800320-40010-3550-160190-220HadfieldBasic730-800340-37025-4060-140210-230Hadfield +Mo, TiadditionBasic730-820350-39030-4560-140210-250Hadfield +Nb, TiadditionBasic740-830350-39030-4570-160210-260Hadfield +V, TiadditionBasic770-880350-40030-4570-160210-230Hadfield +Ceaddition
The microstructure and particularly the grain size are associated with the mechanical properties. A smaller and more homogenous grain size is an indicator of improved mechanical characteristics.
The basic microstructure of ASTM A128 grade A Hadfield steel is determined according to “Metals Handbook”. 9th Edition. Volume 9. Metallography and Microstructures, pages 240 and 241″.
With respect to the basic microstructure, the basic microstructure shown in FIG. 1 presents a reduction of the grain size especially in the priority cooling areas. Nevertheless, this reduction of grain size is not seen in the entire microstructure of the part.
The homogeneity of the grain size is related with the improvement of the mechanical properties. Therefore it would be desirable to have a Hadfield steel in which all its mechanical properties are optimized and its microstructure is austenitic and is as homogenous as possible in grain size.
In this sense, an improvement of the material in this aspect would open up the range of applications of Hadfield, therefore reducing its limitations.
Therefore, improving the Hadfield material is required because the current requirements are more demanding as they call for better performances of the parts in industrial applications which were not previously used or required.
Likewise, Japanese patent no. JP-57-203748-A is known, which describes a composition corresponding to a Hadfield steel that incorporates Hf in its composition, but in high percentages (between 0.1 and 2.5% by weight of the composition) which allow, by means of applying focused heat source (laser, source of electrons, ultraviolet) obtaining magnetized areas in the material, i.e., it is not related with the improvement in the mechanical properties of the Hadfield steel.